Computer problems that have plagued major banks in recent months have struck again, this time crippling Westpac and St George services, including ATMs, and affecting railway customers in New South Wales and Victoria.

"Westpac has experienced an air-conditioning problem at one of its data centres which has triggered a shut-down of our online banking site and some our ATM and eftpos facilities," Westpac said in a statement.

Sorry ... The message Westpac ATM customers received today when they tried to use the bank's ATMs. Photo: Tim Young

In a recent update on their Twitter social media site, Westpac said: ''Systems are recovering, still a little while before they're up again."

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"Please use branch or (telephone) banking, we apologise for inconvenience," the bank said on Twitter.

Eftpos machines at NSW railway stations are affected by this morning's computer glitch. Transport NSW said train travellers should use cash, pay at their destination or pay on return. The disruption extends to regional travel centres and CountryLink on-board services, a spokesman said in an email.

Melbourne-based customer Mark Barrow said he was unable to access money from Westpac through four different accounts and was told he could not withdraw funds from branches.

Mr Barrow said he was alerted to the problem after purchases he made overnight were declined.

"I've spoken to Westpac this morning and they told me none of my cards will work," Mr Barrow said.

'Technical issues'

Westpac earlier tweeted this morning: "We are currently experiencing technical issues. Currently being investigated. Very sorry for inconvenience."

Norm Douglas in the rural town of Forrest in Victoria's south-west, said he first learnt of the problem about 5am trying to make some online purchases, which were declined.

"I run a number of businesses from home and rely on internet banking," he said.

Fairfax also understands the problem could have spread to Adelaide, where people at the Adelaide Aiport were unable to access money from the bank's ATMs.

National Australia Bank's payment system failed in mid-April, preventing hundreds of thousands of customers from receiving pay and forcing the company to open branches over the weekend.

A similar but larger problem brought NAB's system to a grinding halt at the end of 2010, infuriating customers and business owners.

Also, Commonwealth Bank's response to a technology problem at the beginning of March caused its ATMs to allow customers to overdraw their accounts temporarily. Later, the bank was criticised for the heavy-handed tactics it used to recover the money.